Spatial heterogeneity in carbohydrates and their utilisation by microbial communities in the high North Atlantic

Carbohydrates are chemically and structurally diverse, represent a substantial fraction of marine organic matter and are key substrates for heterotrophic microbes. Studies on carbohydrate utilisation by marine microbes have been centred on phytoplankton blooms in temperate regions, while far less is known from high-latitude waters and during later seasonal stages. Here, we combine glycan microarrays and analytical chromatography with metagenomics and metatranscriptomics to show the spatial heterogeneity in glycan distribution and their utilisation by microbes in Atlantic waters of the Arctic during late summer. The composition and abundance of monomers and glycan structures in POM varied with location and depth. Complex fucose-containing sulfated polysaccharides, known to accumulate in the ocean, were consistently detected, suggesting limited degradation by microbes. In contrast, the more labile {beta}-1,3-glucan exhibited a patchy distribution, indicating local variations in primary productivity and rapid utilisation. Metatranscriptomics showed active and dynamic microbial populations that targeted specific glycans. Gene transcription of carbohydrate-active enzymes revealed narrow substrate niches for specialists, involving compounds such as -mannans and alginate, along with the targeting of communal substrates, such as laminarin, by multiple populations. The observed spatial heterogeneity indicates that local biological and physical processes continue to shape the carbohydrate pool during late summer in high latitude waters and microbial populations are active and responsive to such changes.